1. Field of the Invention
This invention relates to a method for the washing of filtering grains which form a filter bed in a filter device.
2. Description of the Prior Art
Generally filter devices are used for the purpose of purifying industrial effluents from iron and steel works and paper and pulp mills and secondarily treated water from sewage works (hereinafter referred to collectively as "raw water"), for example. In these filter devices there are disposed filter beds formed of granular filtering materials such as sand. As the raw water is introduced into the filter device through the upper side thereof and passed through the bed of such filtering grains, water-insoluble solids, oil droplets, etc., suspended in the raw water are retained as the filtration residue and the purified water is released via the lower side of the filter device. During the process of this water purification, part of the filtration residue at times, if not always, forms spheres called "mud balls," which may possibly be retained within the filter bed.
The operation of raw water purification or the operation of filtration is carried out continuously. However, when the amount of said filtration residue increases in the course of continued filtration, the pressure loss occurring in the bed of filtering grains gradually increases to the extent of degrading the efficiency of purification. In this case, the bed of filtering grains is washed by a method described below so as to free it from the filtration residue. The washing is effected by blowing compressed air into the filter device through the lower side thereof to agitate the filter bed vigorously and scrape the deposited filtration residue frictionally off the filtering grains and subsequently giving the filter bed a backwashing with water by causing an upward current of cleansing water to flow through the bed via the lower side of the device. The backwashing loosens the filter bed and fluidizes its individual grains, causing the filtration residue to float up in conjunction with the ascending water and depart from the device. Some filtration residue adheres so fast to the filtering grains that the separation of the adhering filtration residue from the filtering grains requires blowing of compressed air. Mud balls are formed by conglomeration of oily components and suspended solids and they often survive the washing and persist within the filter bed. As the filtration is resumed through the washed filter bed, these mud balls gain in size and, occasionally, break into fine segments and leak into the purified water.
As described above, the washing method known to the art requires the combination of two operations, blowing of compressed air and backwashing with water. For this method, therefore, use of an air compressor and an accumulator is found indispensable.
Since the washing such as is described above fails to provide thorough removal of these mud balls, it often becomes necessary to stop the operation of the filter device and give the filtering grains a washing with water. This interruption has prevented the filter device from being operated continuously at a high efficiency.